In the simplest form, an organic electroluminescent (EL) device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light emitting diodes, or OLEDs. A basic organic EL element is described in U.S. Pat. No. 4,356,429. In order to construct a pixelated OLED device such as is useful, for example, as a television, computer monitor, cell phone display, or digital camera display, individual organic EL elements can be arranged as pixels in a matrix pattern. These pixels can all be made to emit the same color, thereby producing a monochromatic device, or they can be made to produce multiple colors such as a red, green, blue (RGB) device.
The simplest pixelated OLED devices are driven in a passive matrix configuration. In a passive matrix, the organic EL material is sandwiched between two sets of orthogonal electrodes (rows and columns). An example of a passive matrix driven organic EL device is described in U.S. Pat. No. 5,276,380. This approach to producing a pixelated device, however, has several disadvantages. First, only a single row (or column) is illuminated at any given time. Therefore, in order to achieve the desired average brightness for a given frame of video, the row should be illuminated to an instantaneous brightness equal to the desired average brightness multiplied by the number of rows. This results in higher voltages and reduced long term reliability compared to a situation where the pixels are capable of being lit continuously for the entire frame. Second, the combination of high instantaneous current and electrodes that are long and narrow, and therefore have high resistance, results in significant voltage drops across the device. These variations in voltage across the display adversely affect brightness uniformity. These two effects become worse as the size of the display and number of rows and columns are increased, thereby limiting passive matrix designs to relatively small, low resolution displays.
To resolve these problems and produce higher performance devices, recent OLED device designs are typically driven by active matrix (AM) circuitry. In an active matrix configuration, each pixel is driven by multiple circuit elements such transistors, capacitors, and signal lines. This circuitry permits for the pixels of multiple rows to remain illuminated simultaneously, thereby decreasing the required peak brightness of each pixel.
Early active matrix devices, such as those described in U.S. Pat. Nos. 5,550,066, 6,281,634, and 6,456,013, employ a voltage driven type pixel circuit. A voltage drive type active matrix circuit controls the brightness level of the pixels using a voltage data signal. The voltage signal is converted to a current by one or more drive transistors within each pixel. A drive transistor is a transistor which has its source and drain terminals electrically connected between the organic EL element and a power connection (or power line) and which regulates the current flow through the organic EL element in response to a voltage applied to its gate terminal.
These OLED devices are typically mass produced on large substrates with several panels fabricated simultaneously on a single substrate. A typical substrate is made of glass. The transistors are therefore commonly formed in thin film layers of semiconductor material such as silicon. Transistors constructed in thin films of silicon are commonly known as thin film transistors (TFT's). This silicon is typically deposited as an amorphous film. In order to improve the mobility of the silicon, the silicon can be annealed to form polycrystalline silicon, also known as polysilicon. One common process used to perform the annealing is to irradiate the silicon layer with a laser. One such laser annealing process is known as excimer laser annealing (ELA). An example OLED device having laser annealed thin film transistor is described in U.S. Pat. No. 6,548,867.
However, when the transistors are fabricated as thin films, the amount of variation of the properties of the transistors is large. In an OLED device, the brightness of an organic EL element is controlled by the current density flowing through the organic EL element. Variations in the characteristics of the pixel transistors such as mobility and threshold voltage can directly impact the current flowing through the organic EL element, which in turn affects pixel brightness. Variation across the OLED device can result in nonuniform brightness or coloring of the device.
In order to obtain more uniform brightness from an OLED device given variability in the transistor manufacturing process, new designs have been introduced which are current drive type active matrix pixel circuits. A current drive type active matrix circuit controls the brightness level of the pixels using a current data signal. In a current type active matrix pixel circuit, the data signal is in the form of a current signal unlike the voltage signal used in the voltage type active matrix circuits.
Examples of a current type active matrix pixel circuit are the current mirror type pixel circuit disclosed in U.S. Pat. Nos. 6,501,466, 6,535,185, 6,753,654, and U.S. Patent Application Publication 2004/0144978 A2. Current mirror type pixels use a current data source that passes a first current through a first transistor (or conversion transistor) in each pixel. The gate terminal of the first transistor is electrically connected to the gate terminal of a second transistor that has either its source or drain terminal electrically connected to the organic EL element and serves as the drive transistor. This electrical connection to the organic EL element can be either direct as is shown in U.S. Pat. Nos. 6,501,466 and 6,535,185, or indirectly through another transistor as shown in U.S. Patent Application Publication 2004/0144978 A2. The current supplied through the first transistor is then mirrored onto the second transistor by nature of their connected gate terminals thereby establishing a second current. This second current can be equal to the first current or set to some ratio of the first current.
By use of this current mirror design, the need to maintain tight control of the pixel to pixel variability in the characteristics such as threshold voltage and mobility of the drive transistors is reduced. This facilitates use of thin film transistor fabrication techniques and the fabrication of large area devices. However, differences within a pixel between the characteristics of the drive transistor and the transistor having its gate terminal connected to the drive transistor's gate terminal can still result in variable brightness output. In order to reduce this brightness variability of the OLED device, the variability between these two transistors within each pixel should be reduced. Therefore, an improved pixel design with reduced variability is required.